Loans - Summary of Troubled Debt Restructuring (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	$ 179,669	$ 177,984
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	861	914
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	1,346	811
Residential Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	158,136	156,497
Construction & Land Development [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	3,013	3,513
Consumer [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	16,313	16,249
Performing [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	178,975	177,095
Performing [Member] | Commercial [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	861	914
Performing [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	1,346	811
Performing [Member] | Residential Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	157,442	155,608
Performing [Member] | Construction & Land Development [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	3,013	3,513
Performing [Member] | Consumer [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	16,313	16,249
Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	694	889
Nonperforming [Member] | Residential Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	$ 694	$ 889